FIG. 12 is a prior art pressure versus travel diagram for a clutch piston plate in a transmission. For a typical known wet clutch used as a shifting clutch in a transmission a piston is displaced to close the clutch by pressurized oil behind the piston. Elements of a clutch pack for the clutch must be held apart when the clutch is not applied in order to minimize drag and improve fuel economy. This can be accomplished with a return spring. A typical lift-off gap is 2 mm which results in about 0.2 mm of lift-off per friction surface. In order to close the clutch, force is applied to the piston to move the piston into contact with the clutch pack and the force is increased to clamp the clutch pack. As shown in FIG. 12, in this example, a piston plate for the clutch travels 2.0 mm before contacting the clutch pack.
There is a conflict in wet clutch pack design between drag torque and shift time. That is, as the lift-off gap is increased, drag torque drops but shift time goes up. This is because the piston must be moved through the larger lift-off gap. This movement requires a larger amount of oil and a subsequent increase in time to supply the oil. In fact, ⅔ of the oil needed to apply the clutch is used simply to close the lift-off gap. Therefore, known wet clutches are designed with more drag than desired in order to minimize shift time. Furthermore, the point at which the piston contacts the clutch pack can be difficult to determine. The point varies as friction material in the clutch pack sets or wears and varies as a function of time since the piston was last applied. This uncertainty necessitates the added complexity of control algorithms to avoid rough shifts. Because the lift-off gap is so critical, tolerance adjusting methods are used when producing friction packs. These methods can include measuring the assembled pack and installing a select fit washer, or sorting plates and combining those that will give a nominal stack. Both methods add to production cost.